Terminal and method for switching between radio access technologies

ABSTRACT

A method includes determining whether a packet switched (PS) signaling connection is released, the PS signaling connection being associated with a first network corresponding to a first radio access technology (RAT), controlling a switching timing to a second network corresponding to a second RAT according to a type of the PS signaling connection, and transmitting a request for switching to the second network. A terminal includes a controller to determine whether a packet switched (PS) signaling connection is released, the PS signaling connection being associated with a first network corresponding to a first radio access technology (RAT), and to control a switching timing to a second network corresponding to a second RAT according to a type of the PS signaling connection, and a transmitter to transmit a request for switching to the second network.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/653,969, filed on Oct. 17, 2012, and claims priority from and thebenefit under 35 U.S.C. § 119(a) of Korean Patent Application No.10-2012-0043688, filed on Apr. 26, 2012, which is hereby incorporated byreference for all purposes as if fully set forth herein.

BACKGROUND Field

The following disclosure relates to a terminal and method for switchingbetween radio access technologies.

Discussion of the Background

A circuit switched fallback (CSFB) technique may be used to process acircuit switched call by switching from a long-term evolution (LTE)network to a 3rd generation (3G) network or to a 2nd generation (2G)network if the circuit switched call is requested while connecting tothe LTE network, and reverting to the LTE network if the circuitswitched call is terminated. The circuit switched call using the CSFBprocedure (CSFB call) may be processed by enabling fallback from the LTEnetwork to the 3G/2G network for switching between radio accesstechnologies (RATs). This may involve a procedure of registering alocation of a terminal in the 3G/2G network. The subsequent switchingback to the LTE network may be executed based on LTE redirectioninformation included in a 3G radio resource control (RRC) connectionrelease message.

FIG. 1 is a diagram illustrating a process of transitioning to along-term evolution (LTE) network using LTE redirection informationafter terminating a circuit switched fallback (CSFB) call. Referring toFIG. 1, a user equipment or mobile station (UE/MS) 101 supporting LTEcommunication protocols may request a CSFB call, and may release RRCconnection from a LTE network 103 by receiving a RRC connection releasemessage from the LTE network 103. The UE/MS 101 may perform a CSFB callprocess after making a transition to a circuit switched network, such asa wideband code division multiple access (WCDMA) network 105. Afterterminating the CSFB call and releasing the RRC connection with thecircuit switched network, the UE/MS 101 may receive redirectioninformation (“redirection info”) to switch back to the LTE network 103.Hereinafter, the UE/MS 101 may be referred to as a terminal or a mobileterminal.

The LTE redirection information may be used for the switching back fromthe 3G wideband code division multiple access (WCDMA) network to a 4thgeneration (4G) LTE network.

However, few networks supporting a 3G RRC connection release messagewith LTE redirection information in a 3G network exist for variousreasons. For example, if a packet switched session for a datacommunication exists, the redirection information is not sent until thePS session is terminated. Accordingly, suggestions have been made todevelop a system architecture supporting a fast return to an LTE networkfrom a 3G network to an LTE network without waiting for normaltermination of a packet switched (PS) session.

However, in the fast return scheme, if a packet switched (PS) session isfound in a process of closing sessions after processing a call, aterminal supporting the fast return to an LTE scheme may transmit asignaling connection release indication (SCRI) message to a network toclose the corresponding PS session. After the PS session is closed, theterminal may execute a transition to an LTE network.

In the fast return scheme, however, the terminal may transmit the SCRImessage to close an active PS session while waiting for a response for arouting area updating request, thereby the terminal may attempt atransition to the LTE network without receiving the response for therouting area updating request. In this instance, since a context ofswitching between RATs may not be successfully transferred, the locationregistration in an LTE network may fail and voice call and datacommunication failures may occur until a successful locationregistration in the LTE network.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form any part of theprior art nor what the prior art may suggest to a person of ordinaryskill in the art.

SUMMARY

The following description relates to a terminal and method for switchingbetween radio access technologies.

Additional features of the invention will be set forth in thedescription which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention.

Exemplary embodiments of the present invention provide a method forswitching between radio access technologies (RATs) for a terminalincluding determining whether a packet switched (PS) signalingconnection is released, the PS signaling connection being associatedwith a first network corresponding to a first radio access technology(RAT), controlling a switching timing to a second network correspondingto a second RAT according to a type of the PS signaling connection, andtransmitting a request for switching to the second network.

Exemplary embodiments of the present invention provide a method forswitching between radio access technologies (RATs) for a terminalincluding determining a status of a packet switched (PS) signalingconnection associated with a first network corresponding to a firstradio access technology (RAT), controlling a transmission of a signalingconnection release indication (SCRI) message to the first network basedon the status of the PS signaling connection, and transmitting a requestfor switching to a second network corresponding to a second RATdepending upon the transmission of the SCRI message.

Exemplary embodiments of the present invention provide a terminal forswitching between radio access technologies (RATs) including acontroller to determine whether a packet switched (PS) signalingconnection is released, the PS signaling connection being associatedwith a first network corresponding to a first radio access technology(RAT), and to control a switching timing to a second networkcorresponding to a second RAT according to a type of the PS signalingconnection, and a transmitter to transmit a request for switching to thesecond network.

It is to be understood that both forgoing general descriptions and thefollowing detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.Other features and aspects will be apparent from the following detaileddescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate exemplary embodiments of theinvention, and together with the description serve to explain theprinciples of the invention.

FIG. 1 is a diagram illustrating a process of transitioning to along-term evolution (LTE) network using LTE redirection informationafter terminating a circuit switched fallback (CSFB) call.

FIG. 2 is a diagram illustrating a process of transitioning to an LTEnetwork using system information block (SIB) 19 information afterterminating a CSFB call.

FIG. 3 is a diagram illustrating a process of performing a CSFB callaccording to a related art.

FIG. 4 is a diagram illustrating a process of performing a locationregistration after termination of a CSFB call and transitioning to anLTE network according to a related art.

FIG. 5 is a diagram illustrating a process of transitioning to an LTEnetwork using a fast return scheme.

FIG. 6 is a flowchart illustrating a method for switching between radioaccess technologies (RATs) for a terminal according to an exemplaryembodiment of the present invention.

FIG. 7A is a diagram illustrating a method for switching between RATsfor a terminal according to an exemplary embodiment of the presentinvention.

FIG. 7B and FIG. 7C are diagrams illustrating a method for switchingbetween RATs for a terminal according to an exemplary embodiment of thepresent invention.

FIG. 8 is a diagram illustrating a process of a CSFB call processingwith a conditional fast return to LTE function based on a packetswitched session status according to an exemplary embodiment of thepresent invention.

FIG. 9 is a block diagram illustrating a terminal for switching betweenRATs according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The invention is described more fully hereinafter with reference to theaccompanying drawings, in which exemplary embodiments of the inventionare shown. This invention may, however, be embodied in many differentforms and should not be construed as limited to the embodiments setforth herein. Rather, these exemplary embodiments are provided so thatthis disclosure is thorough, and will fully convey the scope of theinvention to those skilled in the art. In the drawings, the size andrelative sizes of layers and regions may be exaggerated for clarity.Like reference numerals in the drawings denote like elements.

It will be understood that when an element is referred to as being“connected to” another element, it can be directly connected to theother element, or intervening elements may be present.

FIG. 1 is a diagram illustrating a process of transitioning to along-term evolution (LTE) network using LTE redirection informationafter terminating a circuit switched fallback (CSFB) call, and FIG. 2 isa diagram illustrating a process of transitioning to an LTE networkusing system information block (SIB) 19 information after terminating aCSFB call.

As shown in FIG. 1 and FIG. 2, a user equipment or mobile station(UE/MS) 101 and 201 may perform a circuit switched fallback procedureswitching from all-IP-based packet switched network, such as the LTEnetwork 103 and 203, to a hybrid type network including a circuitswitched network and a packet switched network, such as the WCDMAnetwork 105 and 205. In a circuit switched fallback procedure, the UE/MS101 and 201 may perform a circuit switched call by switching to acircuit switched network, such as a 3G network including WCDMA networks105 and 205, from a packet switched network, such as an LTE network 103and 203, and, if the circuit switched call is terminated, the UE/MS 101and 201 may perform switching back to the LTE network. The UE/MS 201 maybe referred to as a terminal or a mobile terminal.

Referring to FIG. 1, in the process of switching from the 3G networkback to the LTE network, the switching back may be executed based on LTEredirection information (Redirection info) transmitted with a radioresource control (RRC) message, as shown in operation S130 of FIG. 1.

If a packet switched (PS) session is still open after the circuitswitched call is terminated in the 3G network, in operation S110, thecircuit switched network may wait until the PS session is closed beforetransmitting the redirection info. The circuit switched network mayinclude LTE redirection information in an RRC message indicating thatcircuit switched (CS) session and PS session are closed in operationS120, and may transmit the message with LTE redirection information tothe UE/MS 101 in operation S130. The transition to the LTE network 103may be executed using the received LTE redirection information inoperation S140, and LTE registration may be executed in operation S150.

Referring to FIG. 2, after a CSFB call is terminated in a 3G network,such as the WCDMA network 205, the UE/MS 201 may be in an idle state inthe 3G network and system information for switching back to the LTEnetwork 203 may be obtained in operation S210 if a packet switchedsession is closed. Among the 3G system information, system informationblock 19 (SIB 19) containing Inter-RAT (radio access technology)frequency and priority information may be transmitted to the UE/MS 201in operation S220. The radio access technology (RAT) may indicate aradio technology that enables a wireless communication terminal toaccess a core network via a wireless channel. Examples of the RATinclude Global System for Mobile communication (GSM), Universal MobileCommunication System (UMTS) Terrestrial Radio Access Network (UTRAN),Evolved UTRAN (eUTRAN), and the like, but aspects are not limitedthereto. The UE/MS 201 may execute an LTE reselection based on thesystem information, SIB 19, in operation S230. The transition to the LTEnetwork 203 may be executed based on the received 3G system information,SIB 19, in operation S240, and LTE registration may be executed inoperation S250.

According to the process shown in FIG. 2, an LTE signal strength may bemeasured at a determined cycle in an idle state in the 3G network, andtransitioning to the LTE network may be executed if the signal strengthis higher than a threshold value set in SIB 19.

Thus, after a call is terminated, operations of obtaining SIB 19 systeminformation in an idle state in 3G, measuring an LTE signal strength,and verifying whether the signal strength is higher than the thresholdvalue are performed. Furthermore, as shown in FIG. 1 and FIG. 2, theswitching back procedure is delayed until a PS session associated withthe WCDMA network 205 is closed. Accordingly, transitioning from the 3Gnetwork to the LTE network may be delayed.

FIG. 3 is a diagram illustrating a process of performing a CSFB callaccording to a related art.

Referring to FIG. 3, the call processing procedure for a CSFB call maybe performed via a user equipment (UE)/a mobile station (MS) 301, and anetwork including an evolved NodeB (eNodeB) 303, a base stationsubsystem (BSS)/a radio network subsystem (RNS) 305, a mobilitymanagement entity (MME) 307, a message switching center (MSC) 309, aserving gateway (SGW)/a packet data network gateway (PGW) 311, and aserving general packet radio service support node (SGSN) 313.

Hereinafter, the CSFB call processing procedure is described through asignaling message flow between the UE/MS 301 and the network, includingcall request of an evolved universal mobile telecommunications systemterrestrial radio access network (E-UTRAN) system in LTE and callprocessing of GERAN/UTRAN in WCDMA. GERAN is an abbreviation for aglobal system for mobile communications (GSM)/enhanced data rates forglobal evolution (EDGE).

If a CSFB call is attempted by the UE/MS 301 after transitioning to a 3Gsystem, for example, WCDMA, or to a second generation (2G) system, forexample, GSM, the call may be processed in the 3G/2G network throughoperations S340, S350, S360, and S370. According to a CSFB callprocessing procedure, if a call is requested by the UE/MS 301 aftertransitioning to a 3G/2G network, a connection management (CM) servicerequest message for the call is transmitted in operation S340, and aprocedure for location registration in the 3G/2G network is performed.

The UE/MS 301 may transmit an extended service request (ESR) message tothe MME 307 in operation S320. The ESR message may correspond to amessage that the UE/MS 301 transmits to the MME 307 in response to apaging indication received from the MME 307.

In operation S330, the eNodeB 303 may transmit an LTE radio resourcecontrol (RRC) release message to the UE/MS 301 to perform an inter-RATcell change for fallback to 3G/2G network so that the call may beprocessed in the 3G/2G network. The inter-RAT cell change may includecell change and cell reselection for the UE/MS 301 when a mobile stationis connected to a network.

In operation S340, if a transition to the 3G (e.g., WCDMA)/2G (e.g.,GSM) system is performed for transitioning to a circuit switchednetwork, the UE/MS 301 may continue a call setup process by transmittingthe CM service request message to the MSC 309.

In operation S350, if it is determined that a location area code (LAC)is changed, the UE/MS 301 may attempt location registration for acircuit switched (CS) domain through a location update request message.If it is determined that the LAC is not changed in operation 350,location update request message may not be transmitted.

Generally, a service area of a cellular wireless communication networkmay be divided into location areas, and each location area may includeat least one radio cell. A unique number, also known as a location areacode (LAC), may be assigned to each location area in the network. TheLAC may be used as unique reference information for recognizing alocation of a mobile subscriber, such as the UE/MS 301, and forconnecting an incoming call to the mobile subscriber.

In operation S360, the UE/MS 301 may continue the call processingprocedure regardless of the result of location registration attempted inoperation 350. In operation S370, the UE/MS 301 may perform a procedurefor location registration for a PS domain during the call processing. Asshown in the signaling message flow of FIG. 3, since the CSFB callprocessing is performed by switching between RATs, the procedure forlocation registration may be executed during the CSFB call processing.

FIG. 4 is a diagram illustrating a process of performing a locationregistration after termination of the CSFB call and transitioning to anLTE network according to a related art. Referring to FIG. 4, an LTEsystem may include a UE 401, an MME 403, an MSC/a visitor locationregister (VLR) 405, and a home subscriber server (HSS) 407.

To perform the procedure for location registration in the LTE system,the UE 401 may transmit a tracking area update (TAU) request message tothe MME 403 and receive a TAU accept message from the MME 403 inresponse to the TAU request for subsequent incoming and outgoing callattempts and data access attempts. A tracking area (TA) may refer to aunit area indicating a plurality of adjacent eNodeBs defined as onegroup, and the UE may perform a periodic updating by transmittinginformation about a TA in which the UE is currently located, to theeNodeB, even in an idle state.

After a CSFB call is terminated, a transition to the LTE system may beperformed by switching between RATs as described in operation S240 ofFIG. 2, and the procedure for location registration in the LTE systemmay be executed as described in operation 250 of FIG. 2. FIG. 3illustrates the procedure for location registration through a signalingmessage flow between the UE 401 and the LTE network.

Referring to FIG. 4, if the transition to the LTE system is executedafter the termination of the CSFB call, the UE 401 may determine whetherto perform a location registration process in the LTE network inoperation S410.

The UE 401 may transmit a TAU request message to the MME 403 based onthe determination result in operation S420.

The MME 403 may transmit a TAU accept message to the UE 401 in responseto the TAU request in operation S430. The UE 401 may complete theprocedure for location registration by transmitting a TAU completemessage to the MME 403 in operation S440.

FIG. 5 is a diagram illustrating a process of transitioning to an LTEnetwork using a fast return to LTE scheme.

According to the switching back procedures shown in FIG. 1 and FIG. 2,transitioning to an LTE network may be delayed, if a PS session for datais open, until the PS session is closed according to instructions of thenetwork.

To address this issue, the process illustrated in FIG. 5 suggests aswitching back procedure based on a fast return to LTE scheme to enablea fast transition to LTE network after a call is terminated in a circuitswitched network.

For a fast transition to an LTE network, the network may include a UE501, an eNodeB 503, a BSS/RNC 505, an MME 507, an SGSN 509, an SGW 511,and an MSC/VLR 513, and an operational flow based on the fast returnscheme is described in detail. Referring to FIG. 5, after terminating aCSFB call procedure in operation S520, the BSS/RNC 505 may transmit aradio bearer release message (CS RAB) to the UE 501, and the UE 501 maytransmit a radio bearer release complete message to the BSS/RNC 505 inoperations S525 and S530, respectively. The UE 501 may determine whetherthe terminate call is a CSFB call in operation S535. If the terminatedcall is determined to be a CSFB call and a PS signaling connectionexists in the 3G/2G network, the UE 501 may enable transmission of asignaling connection release indication (SCRI) message for a PS domainto close a PS session when the PS session is open after a CSFB call isterminated. The UE 501 may receive Layer 2 Acknowledgement (L2 Ack) inresponse to the SCRI in operation S545.

If no sessions are open in CS and PS domains or if the L2 Ack isreceived by the UE 501, an RRC connection release message may betransmitted from the BSS/RNC 505 in operation S555. Further, the UE 501may start a connection release timer (CRT) in operation S550 if the L2Ack is received. If the UE 501 receives the RRC connection message, theUE 501 may execute a transition to an LTE network using LTE systeminformation obtained before. Accordingly, even though the UE 501 doesnot receive LTE redirection information from the network, the UE 501 maymake a determination and may perform a transition to the LTE networkusing LTE system information.

The radio bearer release message may correspond to a message that theBSS/RNC 505 transmits to the UE 501 to release a connection of aphysical channel after termination of the call. If the radio bearer isreleased, the physical channel may be changed or released based onwhether the radio bearer can be reused after the release of the radiobearer.

As described above, if the terminated CS call is determined to be a CSFBcall in operation S535 and a PS signaling connection exists, the UE 501may transmit the SCRI message for the PS domain to the BSS/RNC 505 inUTRAN or a universal mobile telecommunications system (UMTS) network inoperation S540.

If the UE 501 receives, from the BSS/RNC 505, Layer2 ACK to the SCRImessage for the PS domain in operation 545, the UE 501 may operate aconnection release timer (CRT) in operation S550. The CRT may allow theUE 501 to execute a transition to LTE within a determined time period.The CRT may correspond to a timer for designating a waiting time givenfor the RRC connection release.

During operation of the CRT, RRC connection release and RRC connectionrelease complete may be performed in operations S555 and S560,respectively, and the UE 501 may execute a transition to the LTE networkbased on LTE system information in operation S570, if the RRC connectionrelease message is received within the determined time period (‘Case1’).

If the operations 555 and 560 are not normally performed until the CRTexpires, the UE 501 may execute an RRC connection release in operationS565, and may execute a transition to the LTE network based on LTEsystem information in operation S570 (‘Case 2’). In order to register inthe LTE network again, the UE 501 may receive system information on E-sUTRAN, establish random access channel (RACH) and RRC connection, andtransmit RRC connection setup complete message. Further, the UE 501 mayperform a combined TAU procedure, and resume a PS session on E-UTRAN. Anexample of the combined TAU procedure is described in 3GPP TS 23.272V11.1.0 (2012-06): 3rd Generation Partnership Project; TechnicalSpecification Group Services and System Aspects; Circuit Switched (CS)Fallback in Evolved Packet System (EPS); Stage 2 (Release 11), which isincorporated herein by reference in its entirety.

However, according to the fast return to LTE scheme, when a PS signalingconnection exists after the termination of a CSFB call, the UE 501transmits the SCRI message for a PS domain as described in operationS540 regardless of whether the UE is waiting for a is response, e.g., aresponse for a routing area updating request. Even though the responsefor the routing area updating request is not received in a 3G network,transitioning to the LTE network is triggered by transmitting the SCRImessage and the subsequent processes illustrated in FIG. 4 areperformed. Throughout the specification, the existence of a PS sessionor a PS signaling connection may refer to a state in which the PSsession or the PS signaling is not released (i.e., the PS session is notclosed). For example, if a PS signaling connection for a locationregistration of a terminal exists, the location registration of theterminal is not completed and further signaling between the terminal andcounterpart core network may be performed to complete the locationregistration, e.g., routing area update.

Since a context about switching between RATs may not be exchangedsuccessfully in the network, a procedure for location registration inthe LTE network may fail (#10—IMPLICITLY_DETACHED) and operation 430 ofFIG. 4 may not be performed. As a result, an incoming and outgoing callfailure and a data access delay may occur until the next locationregistration attempt and registration.

To address this issue, exemplary embodiments of the present inventionprovide a method for controlling a transmission time of a packetswitched signaling connection release indication (PS SCRI) message basedon a determination of whether a PS session for location registration isopen and/or a PS session for data communication is open.

FIG. 6 is a flowchart illustrating a method for switching between RATsfor a terminal according to an exemplary embodiment of the presentinvention. A communication method of switching between RATs for aterminal in a communication system including a first network using afirst RAT and a second network using a second RAT is described withreference to FIG. 6. The second network may comprise a packet switchednetwork.

Referring to FIG. 6, after a CSFB call is terminated in operation S610,the terminal may verify whether a PS session for registering a locationof the terminal is active in operation S620. Further, before verifyingwhether the PS session for registering the location of the terminal isopen, the terminal may verify whether a PS session for datacommunication is open.

If it is verified that the PS session for registering the location ofthe terminal is open in operation S620, the terminal may verify whethera procedure for location registration is completed in operation S630.The procedure for location registration may include a procedure forregistering the location of the terminal in the first network byswitching from the second RAT to the first RAT, and a procedure forregistering a new location of the terminal in the first network bychanging a routing area code (RAC) during a call processing procedure.The change of the RAC may include a change of a routing area caused bymovement of the terminal. The first network may include the WCDMAnetwork, and the second network may include the LTE network, forexample.

If it is verified that the PS session for registering the location ofthe terminal is absent in operation S620, the terminal may transmit anSCM message to the first network in operation S650, and may execute atransition from the first network to the second network in operationS660.

In operation S640, the terminal may determine a transmission time of theSCM message for closing the PS session based on the verified result inoperation S630. The terminal may determine the transmission time of theSCM message based on the determination of whether the procedure forlocation registration is completed.

Further, the terminal may determine whether to perform closing of the PSsession based on the determination of whether the procedure for locationregistration is completed. If the procedure for location registration iscompleted and the PS session for data is open, the terminal maydetermine to perform closing of the PS session. If it is determined thatclosing of the PS session is to be performed, the terminal may transmitthe SCRI message to the first network.

If it is determined not to perform closing of the PS session, theterminal may execute a transition from the first network to the secondnetwork using the second RAT system information obtained before settingup a CSFB call in operation S660.

FIG. 7A is a diagram illustrating a method for switching between RATsfor a terminal according to an exemplary embodiment of the presentinvention.

‘PS signaling connection’ mentioned herein may represent an existence ofan active PS session including a series of procedures for processing adata communication through a PS domain and/or a series of procedures forlocation registration.

Further, the ‘procedure for location registration’ may include aprocedure for registering a location in a first network, e.g., a 3Gnetwork, by enabling fallback from a second network, e.g., an LTEnetwork, to the first network for switching between RATs, and aprocedure for registering a location by changing an RAC as described inoperation S370 of FIG. 3. The change of the RAC may include a change ofa routing area caused by a movement of the terminal while the 3G call isconnected.

Referring to FIG. 7A, if a CSFB call is terminated in operation S705,the terminal may verify whether a PS session is open in operation S710.The determination of whether the PS session is open may be verifiedbased on a determination of whether a PS signaling connection message istransmitted.

If the CSFB call is terminated, the terminal may verify whether a PSsignaling connection message is communicated in operation S710. If thePS signaling connection message is absent, the terminal may wait for RRCconnection release or wait for the expiration of CRT and may execute atransition to an LTE network using LTE information obtained before ifthe RRC connection is released or the CRT is expired in operation S730.

If it is verified that the PS signaling connection message iscommunicated and the PS signaling connection exists in operation S710,the terminal may verify whether the PS session for data communication, aPS signaling connection message for data communication, exists inoperation S715.

If the PS signaling connection message for data exists in operationS715, the terminal may request closing of the PS session. In suchinstance, the terminal may request the closing of the PS sessionregardless of whether a procedure for location registration isperformed.

If transitioning to the LTE network is executed in a state where theprocedure for location registration is not completed in 3G/2G network,operation 430 illustrated in FIG. 4 may fail during locationregistration illustrated in FIG. 1 and FIG. 2.

To address this issue, if the PS session for data exists in operationS715, the terminal may verify whether a PS session for locationregistration, a PS signaling connection message for locationregistration, exists, before closing the PS session, in operation S720.Further, if the PS session is open and the PS signaling connectionmessage for data is absent in operation S715, the terminal may verifywhether the PS signaling connection message for location registrationexists in operation S735.

If the PS signaling connection message for location registration isabsent in operation S720, the terminal may transmit, to the network, aPS SCM message indicating the closing of the PS session to close the PSsession for a fast return to LTE in operation S725. The terminal mayperform the fast return to LTE based on operations S550, S555, S560,S565, and S570 illustrated in FIG. 5.

That is, if the RRC connection is released or the CRT expires, theterminal may execute a transition to an LTE network in operation S730.

For example, transmission of the PS SCRI message by the terminal maycorrespond to processing of operation S540 of FIG. 5, and if theterminal receives Layer2 ACK in response to the SCRI message inoperation S545 of FIG. 5, the terminal may operate the CRT.

Before the CRT expires, if the terminal receives an RRC connectionrelease message from, for example, a BB S/RNS and transmits an RRCconnection release complete message to the BBS/RNS in response to theRRC connection release message, the RRC connection may be released andthe terminal may execute a transition to the LTE network. The RRCconnection release may correspond to the close of CS and PS sessions.

If the RRC connection is not released until the CRT expires, theterminal may release the RRC connection and execute a transition to theLTE network after the CRT expires.

If the PS signaling connection message for data is absent in operationS715, the terminal may verify whether the PS signaling connection forlocation registration exists in operation S735. If the PS signalingconnection for location registration exists, e.g., the procedure forlocation registration is not completed, the terminal may not transmitthe PS SCRI message and may wait until the procedure for locationregistration is completed in operation S740.

The terminal may verify whether the procedure for location registrationis completed in operation S745.

If it is verified that the procedure for location registration is notcompleted in operation S745, the terminal may perform the operation S740again and may not transmit the PS SCRI message and may wait until theprocedure for location registration is completed.

If it is verified that the procedure for location registration iscompleted in operation S745, the terminal may determine whether toperform closing of the PS session in operation S750.

If the procedure for location registration is completed and datacommunication is not active, the network may perform the signalingconnection release for the PS domain, and the terminal may not transmitthe PS SCM message. However, if the PS session for data is open, sincethe terminal may not execute a transition to LTE, the terminal maydetermine whether to transmit the PS SCRI message. Thus, the terminalmay determine again whether to request closing of the PS session bytransmitting the PS SCRI message in operation S750.

The terminal may determine whether to transmit the SCM message for thePS domain after termination of the CSFB call based on a PS sessionstatus in the connected network. In order to determine the PS sessionstatus, the terminal may verify whether the PS session for data is openand/or whether the PS session for location registration is open.Further, if the PS signaling connection for location registrationexists, the terminal may defer transmitting the PS SCRI message. If thePS session is open at the completion time of location registration, theterminal may transmit the PS SCRI message.

If the PS session is closed according to instructions of the network,the terminal may not transmit the PS SCRI message and wait for RRCconnection release since the PS session is already closed. Further, ifprocessing of the RRC connection release message is completed, theterminal may execute a transition to an LTE network using LTE systeminformation obtained before, even though the terminal is not providedwith LTE redirection information.

The fast return to LTE scheme may provide a process for switching theRAT from the 3G network to the LTE network after the CSFB call isterminated without considering specific PS session status andtransmission of the PS signaling connection message. Since processing ofthe PS signaling connection release message is performed in theconsideration of a PS session for data communication only and locationregistration for the PS domain is not considered, the aforementionedproblems and issues may occur in the fast return to LTE scheme.

However, according to the exemplary embodiment of the present invention,since the terminal may verify whether the PS session for data is openand/or whether the PS session for location registration is open and maytransmit the SCRI message for the PS domain based on the verifiedresult, the terminal may execute a transition to the LTE network morestably and faster than a terminal without the fast return to LTEfunction.

Hereinafter, operations will be described in further detail.

If a CSFB call is terminated in operation S705, the terminal may verifywhether a PS signaling connection exists in operation S710. If the PSsignaling connection is absent, the terminal may wait for RRC connectionrelease, and if the RRC connection is released, the terminal may executea transition to the LTE network using LTE information obtained before inoperation S730.

As described above, if the PS session is closed without consideringwhether a procedure for location registration is performed, locationregistration failures may occur during a registration in the LTEnetwork. That is, since transitioning to the LTE network may be executedin a state where a procedure for location registration is not completedin a 3G/2G network, operation S430 of FIG. 4 may fail during locationregistration to the LTE network.

If it is determined that the PS signaling connection exists in operationS710, the terminal may verify whether a PS session for data exists inoperation S715. If the PS session for data exists, the terminal mayverify whether a PS signaling connection for location registrationexists before closing the PS session in operation S720.

Further, if it is determined that the PS signaling connection for datais absent in operation S715, the terminal may verify whether a PSsignaling connection for location registration exists in operation S735.

If it is determined that the PS signaling connection for locationregistration exists in operation S735, the terminal may not transmit aPS SCRI message and may wait until the procedure for locationregistration is completed in operation S740. If it is determined thatthe PS signaling connection for location registration is absent inoperation S735, the terminal may transmit the PS SCM message inoperation S725.

The operation S725 may correspond to an operation in which the terminaltransmits the PS SCM message to the network to close the PS session forthe fast return to LTE.

If the terminal transmits the PS SCRI message in operation S725, theterminal may wait for Layer2 ACK in response to the PS SCM message, and,if the terminal receives the Layer2 ACK, the terminal may operate a CRTin operation S730. If the RRC connection is released before the CRTexpires, the terminal may execute a transition to an LTE network usingLTE system information obtained before. The LTE system information maybe obtained before the connection to the circuit switched network, e.g.,3G/2G network. If the RRC connection is not released until the CRTexpires, the terminal may release the RRC connection without receivingRRC connection release message and execute a transition to the LTEnetwork.

If the PS signaling connection for location registration is notcompleted, the terminal may repeatedly perform operation S740 andoperation S745. The terminal may not transmit the PS SCRI message andmay wait until the procedure for location registration is completed, andmay verify whether the procedure for location registration is completed.If the procedure for location registration to the 3G/2G network is notcompleted, the terminal may perform the operation S740 again whiledeferring transmitting the PS SCM message. If the procedure for locationregistration is completed, the terminal may perform operation S750.

If the procedure for location registration is completed and datacommunication is not activated in operation S745, the network mayinstruct the terminal to release the signaling connection for the PSdomain, and the terminal may not transmit the PS SCM message since PSsessions are closed. If the PS session for data exists, the terminal maynot execute the transition to the LTE network immediately. In order todetermine whether to transmit the PS SCM message, the terminal maydetermine again whether to perform closing of the PS session or to waitnormal PS session closing in operation S750. If it is determined thatthe PS signaling connection for data needs to be closed in operationS750, the terminal may perform operation S725. However, if it isdetermined that the PS signaling connection for data does not have to beclosed, the terminal may perform operation S730 to attempt an immediatetransition to the LTE network.

FIG. 7B and FIG. 7C are diagrams illustrating a method for switchingbetween RATs for a terminal according to exemplary embodiments of thepresent invention. Referring to FIG. 7B and FIG. 7C, a terminal mayprocess and terminate a call, e.g., a CSFB call, in operation S705. Inoperation S710, the terminal may determine whether a PS signalingconnection is open. If it is determined that a PS signaling connectionis open, the terminal may determine whether a PS signaling connectionfor location registration exists in operation S720. If it is determinedthat a PS signaling connection is not open as determined in operationS710, the terminal may perform transition into LTE network using LTEinformation in response to a receipt of an RRC connection releasemessage from a 3G/2G network or in response to an expiration of CRTtimer in operation S730. In operation S720, the terminal may determinewhether a PS signaling connection for location registration exists,e.g., Routing Area Update signaling. If it is determined that the PSsignaling connection for location registration does not exist asdetermined in operation S720, the terminal may transmit a PS SCRImessage to the 3G/2G network in operation S725 and may perform operationS730. If it is determined that the PS signaling connection for locationregistration exists as determined in operation S720, the terminal maydefer the transmission of the PS SCRI in operation S740 until theprocedure of the location registration is completed and the PS signalingconnection for location registration is closed in operation S745. If itis determined that the procedure of the location registration iscompleted in operation S745, the terminal may determine whether a PSsignaling connection for data exists other than the PS signalingconnection for location registration in operation S747.

Referring to FIG. 7B, if a PS signaling connection for data exists asdetermined in operation S747, the terminal may perform operation S725 torequest closing of the PS signaling connection for data for fast returnto LTE. If the PS signaling connection for data does not exist asdetermined in operation S747, the terminal may perform the transitioninto LTE network using LTE information without waiting for an RRCconnection release message or a CRT timer expiration in operation S730.Further, if the PS signaling connection for data does not exist asdetermined in operation S747, the terminal may perform the transitioninto LTE network using LTE information in response to a receipt of anRRC connection release message from a 3G/2G network or in response to anexpiration of a timer started after the completion of the procedure forlocation registration in operation S730.

Referring to FIG. 7C, if a PS signaling connection for data exists asdetermined in operation S747, the terminal may determine whether toperform closing of the PS signaling connection for data in operationS750. The determination in operation S750 may be performed based on atype of PS signaling connection for data. If the terminal determines toperform closing of the PS signaling connection for data as determined inoperation S750, the terminal may perform operation S725. If the terminaldetermines not to perform closing of the PS signaling connection fordata as determined in operation S750, the terminal may performtransition into LTE network using LTE information without waiting for anRRC connection release message or a CRT timer expiration in operationS730.

FIG. 8 is a diagram illustrating a process of a CSFB call processingwith a conditional fast return to LTE function based on a packetswitched session status according to an exemplary embodiment of thepresent invention.

Referring to FIG. 8, in operation S810, after a call is terminated,UE/MS 801 may verify whether a PS session for data is open and whether aPS session for location registration is open, and may determine whetherto transmit a PS SCM message based on the verified result. The ‘PS SCM’message may correspond to a message transmitted to the network to closethe PS session. The UE/MS 801 may be referred to as a terminal or amobile terminal.

In operation S820, the UE/MS 801 may determine whether to execute atransition to an LTE network based on a determination whether the PSsession is closed. The terminal may verify whether an RRC connection isreleased, e.g., whether CS and PS sessions are closed, or whether a CRTexpires. The UE/MS 801 may start CRT if the UE/MS 801 transmits the SCRIor the UE/MS 801 receives Layer2 acknowledgement (ACK). If the UE/MS 801fails to receive Layer2 acknowledgement (ACK) or RRC connection releasemessage from the network until the CRT expires, the UE/MS 801 mayrelease the RRC connection in order to perform transitioning to the LTEnetwork using a conditional fast return to LTE function. The conditionalfast return to LTE scheme is conditional since the UE/MS 801 determinesthe fast return depending upon one or more criteria, e.g., PS sessionfor location registration status and the UE/MS 801 controls thetransmission of the SCM.

If the terminal receives a determination result about transitioning tothe LTE network from the 3G/2G network, the terminal may execute atransition to the LTE network in operation S840, and may execute an LTEregistration in operation S850.

FIG. 9 is a block diagram illustrating a terminal for switching betweenRATs according to an exemplary embodiment of the present invention.Referring to FIG. 9, the terminal for switching between RATs in acommunication system including a first network using a first RAT and asecond network using a second RAT may include a first verifying unit910, a second verifying unit 920, a transmission time determining unit930, a transmitting unit 940, and a transition unit 950. The first RATmay include, for example, WCDMA, and the second RAT may include, forexample, an LTE network. The terminal may include one or moreprocessors, memories, wireless communication modem chips, antennas, andthe like to implement operations performed by the first verifying unit910, the second verifying unit 920, the transmission time determiningunit 930, the transmitting unit 940, and/or the transition unit 950

The first verifying unit 910 may verify whether a PS session forregistering a location of the terminal exists if a call of a CSFB forswitching between the RATs is terminated.

The first verifying unit 910 may verify whether a procedure forregistering the location is completed. Here, the procedure forregistering the location may include a procedure for registering thelocation of the terminal in the first network after switching from thesecond RAT to the first RAT, and a procedure for registering a newlocation of the terminal in the first network by changing an RAC in astate in which the call is connected. The change of the RAC may includea change of a routing area caused by a movement of the terminal whilethe 3G network call is connected.

The second verifying unit 920 may verify whether a PS session for dataexists.

The transmission time determining unit 930 may determine, based on theverified result of the first verifying unit 910 and/or the secondverifying unit 920, a transmission time of an SCRI message for closingthe PS session

The transmission time determining unit 930 may determine thetransmission time of the SCRI message based on a determination ofwhether the procedure for registering the location is completed.

Further, the transmission time determining unit 930 may include adetermining unit 935 to determine whether to perform closing of the PSsession based on whether the procedure for registering the location iscompleted. If the procedure for registering the location is completedand the PS session for data communication exists, the determining unit935 may determine to perform the closing of the PS session. Further, thedetermining unit 935 may determine whether to perform the closing of thePS session according to a type of the PS session for data other than thePS session for registering the location.

The transmitting unit 940 may transmit the SCM message if it isdetermined to perform the closing of the PS session.

The transition unit 950 may execute a transition from the first networkto the second network using the second RAT system information obtainedbefore setting up the CSFB call if it is determined that closing of thePS session is not necessary.

The exemplary embodiments of the present invention may be recorded innon-transitory computer-readable media including program instructions toimplement various operations embodied by a computer. The media may alsoinclude, alone or in combination with the program instructions, datafiles, data structures, and the like. The media and program instructionsmay be those specially designed and constructed for the purposes of thepresent invention, or they may be of the kind well-known and availableto those having skill in the computer software arts. Examples ofnon-transitory computer-readable media include magnetic media such ashard discs, floppy discs, and magnetic tape; optical media such as CDROM discs and DVD; magneto-optical media such as floptical discs; andhardware devices that are specially configured to store and performprogram instructions, such as read-only memory (ROM), random accessmemory (RAM), flash memory, and the like. Examples of programinstructions include both machine code, such as produced by a compiler,and files containing higher level code that may be executed by thecomputer using an interpreter. The described hardware devices may beconfigured to act as one or more software modules in order to performthe operations of the above-described embodiments of the presentinvention.

According to the exemplary embodiments, a terminal may determine, basedon a verification result about whether a PS session for registering alocation of the terminal exists, a transmission time of an SCM messagefor closing the PS session, thereby preventing an incoming and outgoingcall failure and a data access delay that may occur when releasing a PSsignaling connection.

According to the exemplary embodiments, a terminal may determine, basedon a determination whether a procedure for registering a location of theterminal is completed, a transmission time of an SCRI message, so thatthe terminal may execute a stable and fast transition to an LTE networkregardless of whether the terminal is provided with LTE redirectioninformation from a releasing network.

It will be apparent to those skilled in the art that variousmodifications and variation can be made in the present invention withoutdeparting from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A communication method comprising: determining that a call using afirst network is terminated; in response to determining that the callusing the first network is terminated, determining that a signalingconnection for location registration exists, the signaling connectionbeing associated with the first network; in response to determining thatthe signaling connection for location registration exists, completingprocedures for location registration; in response to completing theprocedures for location registration, transmitting a signalingconnection release indication (SCRI) message; and switching from thefirst network to a second network.
 2. The method of claim 1, wherein thefirst network corresponds to a first radio access technology (RAT) andthe second network corresponds to a second RAT.
 3. The method of claim2, wherein the second network is a long-term evolution (LTE) network. 4.The method of claim 3, wherein the first network is a wideband codedivision multiple access (WCDMA) network.
 5. A communication apparatuscomprising: a processor; and a memory operably coupled to the processor,wherein the processor, when executing program instructions stored in thememory, is configure to: determine that a call using a first network isterminated; in response to determining that the call using the firstnetwork is terminated, determine that a signaling connection forlocation registration exists, the signaling connection being associatedwith the first network; in response to determining that the signalingconnection for location registration exists, complete procedures forlocation registration; in response to completing the procedures forlocation registration, cause the apparatus to transmit a signalingconnection release indication (SCRI) message; and cause the apparatus toswitch from the first network to a second network.
 6. The communicationapparatus of claim 5, wherein the first network corresponds to a firstradio access technology (RAT) and the second network corresponds to asecond RAT.
 7. The communication apparatus of claim 6, wherein thesecond network is a long-term evolution (LTE) network.
 8. Thecommunication apparatus of claim 7, wherein the first network is awideband code division multiple access (WCDMA) network.
 9. A device fora mobile terminal comprising: a processor; and a memory operably coupledto the processor, wherein the processor, when executing programinstructions stored in the memory, is configure to: determine that acall using a first network is terminated; in response to determiningthat the call using the first network is terminated, determine that asignaling connection for location registration exists, the signalingconnection being associated with the first network; in response todetermining that the signaling connection for location registrationexists, complete procedures for location registration; in response tocompleting the procedures for location registration, cause the mobileterminal to transmit a signaling connection release indication (SCRI)message; and cause the mobile terminal to switch from the first networkto a second network.
 10. The device of claim 9, wherein the firstnetwork corresponds to a first radio access technology (RAT) and thesecond network corresponds to a second RAT.
 11. The device of claim 10,wherein the second network is a long-term evolution (LTE) network. 12.The device of claim 11, wherein the first network is a wideband codedivision multiple access (WCDMA) network.